The invention is generally concerned with digital data communication, remote control and remote measurement networks. The invention is particularly concerned with local networks for communicating packets of data using a carrier, more particularly networks dedicated to remote control and remote measurement functions.
In the context of the invention, the term local networks will be used for networks of this kind able to transfer data between any kind of addressable modules using at least one non-dedicated transmission line, these modules being adapted to serve as interfaces between actuators, sensors or central logic units and the aforementioned network, in order to implement said remote control and remote measurement functions.
In one embodiment a network of this kind is used for interchanging data between at least one central unit and any number of actuators, such as control relays, for example, or sensors, the central unit being connected to a so-called "communicator" module whereas the actuators or sensors are connected to so-called "satellite" modules.
In another embodiment, any number of networks as defined hereinabove are juxtaposed so that it is possible to implement in this way a network providing for communication between any number of central units and sensors or actuators.
In these embodiments the transmission network may be a network dedicated to data or a power transfer network such as the 220 V 50 Hz electrical mains supply or any form of power transmission network (12 V, 48 V, etc).
At the time of writing there exist two major families of local networks: local networks using a carrier and local networks not using any carrier.
Generally speaking, the problem to be overcome is that relating to possible conflict between information originating simultaneously from numerous modules. These conflicts are usually referred to as "collisions" of information. The man skilled in the art is well aware that as soon as there occurs simultaneous transmission of two streams of information addressed to the same receiver the latter has difficulty in recognizing the information from each of the transmitters in question. In practice this results in system blockages with all the easily imagined consequences in networks intended either for remote control or for remote measuring applications.
In practice, three types of network have been designed to overcome this problem.
In a first type of network, using carriers exclusively, a specific frequency is assigned to each module. The modules are activated only on receiving a signal at the frequency which has been assigned to them.
This type of network is limited by the fact that an assignable frequency band has to be divided into a number of groups of frequencies, the limitation occuring as a result of such division.
In a second type of network using either no carrier or a carrier at one frequency only, each module initially listens to establish whether there is another module transmitting.
This type of network is also limited, not only as to the number of stations that can be installed, but above all as to the number of stations that can transmit at a given time. This type of network is based on the principle that transmissions occur at random, in other words that during a given time interval the probability that any station or module will transmit is the same, although this is rarely the case is practice: in a remote control or remote measurement network, it is not rare for it to be always the same actuator or the same sensor which is used during a given time interval. During this time interval, there will therefore be a "preference" for this module which will send the longest packets. If two other modules are waiting during this time interval, there is a clear risk of these two modules transmitting at the same time as soon as the "preferred" module stops transmitting.
The third type of network uses no carrier or a carrier at a single frequency and is based on a method consisting in defining time intervals during which the stations or modules can transmit information.
This type of network is essentially limited in two ways. On the one hand, the speed of the network is reduced by virtue of its inherent design. When there are numerous time intervals assigned, there is necessarily a long waiting time for a station transmitting in a given time interval since it must wait for the predetermined time before transmitting.
On the other hand, this type of network has a centralized structure which is particularly limiting from the sequencing point of view. There is, in fact, a synchronization clock.
In certain implementations the clock is the frequency of the mains electrical power supply (50 Hz) which imposes the use for these local data communication networks of alternating current distribution networks.